Use openshift/minimal-registry as a Docker registry to speed up the demo

The `mfojtik/registry` docker file is here:

https://github.com/mfojtik/docker-registry/blob/caf4d6d211ff369521da80f6272ad1f2575a1401/contrib/golang_impl/Dockerfile

However, since it is using the `scratch` image, it needs to be build from the `docker-registry` repo.